Generally, CATV transmission cables are connected to subscribers via multi-tap distribution apparatuses. CATV stations transmit through the cables not only television signals and control signals but also currents for operating main line amplifiers. Therefore, some distribution apparatuses are designed for passing electrical currents therethrough.
Handling instructions and precautions for equipment connected to the transmission cable are not often followed at subscribers' homes. For example, equipment may be inappropriately installed or tampered with by children, which often causes a short circuit. If a short circuit occurs in a secondary line branching out of a terminal of a branch circuit of a distribution apparatus or in terminal equipment connected to the secondary line, it can cause damage not only to the distribution apparatus to which the concerned secondary line is connected but also to the circuits of the equipment installed in the main line.
A fuse or bimetal device is sometimes interposed in the current transmission circuit of a distribution box to block the transmission of a large current. However, replacing a fuse to restore the flow of currents is a time-consuming task as it requires opening the distribution apparatus. On the other hand, the problem with the bimetal device is that, having a high calorific value, this device tends to raise the temperature in the distribution apparatus and hence change the characteristics of the ferrite components inside.